For decades, the realization of broadband, high power monolithic microwave integrated circuit (MMIC) power amplifiers has posed a significant challenge to microwave design and systems engineers, mainly due to limitations imposed by the electrical and thermal properties of GaAs transistor technology. Recently, broadband power amplifier modules consisting of MMIC power amplifiers, matching networks, and control circuits have emerged to meet the demand of power amplification with multi-frequency band coverage. One such design is the single module implementation created by integrating multiple MMIC narrow band power amplifier chips with corresponding matching circuits, controlled by switches for frequency band tuning. However, such an implementation requires multiple MMIC chips, makes the biasing circuit more complicated and fails to decrease the number of components used in the matching circuits.
In order to simplify the design of broadband power amplifier modules, alternative circuit topologies have been proposed. These include, for example, balanced amplifiers, distributed amplifiers, feedback amplifiers, and amplifiers with variety of matching networks. However, each circuit topology and/or matching network has associated advantages and disadvantages that must be considered depending on the particular application.
While a balanced amplifier is a good candidate to meet broadband requirements, the quarter wavelength sizing of the couplers is usually not practical in MMIC's—especially at the low gigahertz frequency range used in conventional wireless handset communications. Distributed amplifiers, on the other hand, can obtain a broad bandwidth and facilitate load matching, but suffer from low gain, low efficiency, and a relatively large chip size. Feedback amplifiers designed as broadband amplifiers have a relatively small chip size, their gain is low at microwave frequencies and their efficiency is compromised when resistive feedback is used. Alternatively, traditional amplifiers (e.g., common source and common emitter topologies) with lossy matching networks can be used to trade off the power gain for better gain flatness over a wider frequency range. On the other hand, in a low-loss matching design, synthesizing the components with realistic values within a broad frequency band is extremely difficult.
Since the output matching network plays a definite role in determining the performance of power amplifiers, such as power gain, power added efficiency (PAE), bandwidth and linearity, a great deal of design effort has been focused on its implementation with the aforementioned MMIC power amplifiers circuit topologies. In order to make the output matching network reconfigurable for various operating frequencies, the network is often made off-chip with tunable components. Not only can the PAE of the broadband power amplifiers be improved by tuning the components value in the output matching network as illustrated in S. Kim, J. Lee, J. Shin, B. Kim, “CDMA handset power amplifier with a switched output matching circuit for low/high power mode operations,” in IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, Jun. 2004 , pp. 1523-1526, A. C. Cotler, E. R. Brown, “The feasibility of a variable output matching circuit in a high-power SSPA,” in IEEE Radio and Wireless Conference, August 2002, pp. 189-191, and J. J. Yao, C. W. Seabury, D. R. PehIke, J. L. Bartlett, J. L. Julian, M. C. F. Chang, H. O. Marcy, K. D. Pedrotti, D. Mehrotra, “Integrated tunable high efficiency power amplifier,” U.S. Pat. No. 6,232,841, May 15, 2001, but also broadband matching can be realized by using two output matching routes. Furthermore, harmonic tuning through the use of photonic band-gap (PBG) and defected ground structure (DGS) at the output of the power amplifier have been proposed recently for broadband matching as well. However, these designs will suffer from low PAE, power gain or large chip size in typical broadband power amplifiers.
Thus, broadband amplifiers capable of overcoming the disadvantages of previous designs are needed.